The invention relates to low-melting lead-free solder glass.
Solder glass for making a glass-metal connection for connecting two glass parts with each other, for sealing, has been known for many years.
In the past the desired low melting temperatures and good processing properties were obtained by including large amounts of lead oxide in solder glass. Currently lead-free solder glasses are increasingly being developed in which lead oxide is replaced by bismuth oxide because of the toxicity of lead and because of legal requirements, e.g. the ROHS guidelines (“Limitations on the Use of Certain Dangerous Materials in Electrical and Electronic Units”, EG Guidelines 2002/95/EG of Jan. 27, 2003).
U.S. Pat. No. 5,326,591 describes a glass with a composition comprising (in wt. % on an oxide basis) 0-9, SiO2; 10-33, B2O3; 10-32, ZnO and 35-77, Bi2O. This solder glass has a high melting temperature of about 550° C.
JP 92 68 026 A describes a glass with a composition comprising (in wt. % on an oxide basis) 0-8, SiO2; 5-35, B2O3; 0-9, ZnO; 36-80, Bi2O3; 5-40, BaO+SrO. However because of the high alkaline earth metal oxide content this glass has a tendency to crystallize.
U.S. Pat. No. 6,778,355 B2 discloses a solder glass for joining two halves of a magnetic head, which has a composition comprising (in wt. % on an oxide basis) 0.5-14, SiO2; 3-15, B2O3, 4-22, ZnO; 55-90, Bi2O3, 0-4, Al2O3; 0-5, alkali metal oxide and 0-15 alkaline earth metal oxide, in which a weight ratio of ZnO/B2O3 is 0.8 to 2 and a weight ratio of SiO2/Al2O3 is not less than 2. This glass has the disadvantage that it contains a large portion of alkali metal and/or alkaline earth metal oxides at the desired low processing temperatures of 455° C. to 550° C., so that the glass melt produced during glass production is very reactive, foamy and corrosive, which leads to high production costs due to its reaction with conventional platinum melt vessels.